There is increasing interest in providing wireless systems in which the functions of a conventional base station are divided between two or more nodes. Baseband processing of wireless signals is performed by a digital unit (DU). Radio frequency processing is performed by a radio unit (RU). Radio frequency signals are transmitted and received by an antenna. The general term for this kind of system is Radio over Fibre (RoF). Signals are carried between the two nodes over an optical transmission link. Signals carried between the two nodes are called fronthaul signals.
In a digital Radio over Fibre system, the radio unit (RU) is located remotely from the digital unit (DU) and is typically called a Remote Radio Head (RRU). The DU and RRU are connected by an optical link. The DU outputs digital values, such as in-phase and quadrature (IQ) values. Data is carried over the optical link in digital form to the RRU. The RRU performs digital-to-analog conversion, and may perform RF functions, for example up-conversion to RF or filtering.
In an analog Radio over Fibre system, the radio unit (RU), and optionally, digital unit (DU), is located at a first node. The antenna is located at, or connected to, the second node. An optical link connects the first node to the second node. In the downlink direction, the first node sends signals over the optical link in analog form at radio frequency (RF) or an intermediate frequency (IF). At the second node, the analog domain signals are received over the optical link, converted to electrical form, and either sent directly to an antenna for transmission, or repositioned in frequency and then transmitted. In the uplink direction wireless signals are received at the antenna. The received signals may be used to modulate an optical transmitter or repositioned in frequency and then used to modulate an optical transmitter. At the first node, signals are converted to the electrical domain and then processed by the radio unit (RU) and digital unit (DU).
Radio over Fibre is particularly attractive where a large number of small cell sites are needed, such as in-building applications. RoF can also be used across larger areas as an alternative to conventional monolithic base stations and backhaul.
Radio over Fibre has an advantage of consolidating much of the base station signal processing at one location. This has an advantage of easier servicing and upgrading of the single location compared to visiting a large number of individual cell sites. The pooling of processing resources makes it possible to share resources between cell sites of the base station.
Analog Radio over Fibre further reduces the amount of equipment required at, or near to, the antenna. It can also reduce latency and reduce power consumption. While analog RoF has some advantages, transmission across the optical link in the analog domain means the system is more prone to the cumulative effects of noise and device nonlinearities, and crosstalk arising from impairments in the optical link.